Method for forming of a carbide layer of a V-a group element of the periodic table on the surface of an iron, ferrous alloy or cemented carbide article

ABSTRACT

A method for forming a carbide layer of a V-a group element V, Nb or TA of the periodic table on the surface of an iron, ferrous alloy or cemented carbide article in a treating molten bath, comprising heating a mixture of boric acid or a borate and a chloride of said V-a group element of the periodic table to its fusing state and immersing the article in the treating molten bath of said mixture, thereby forming a very hard carbide layer of said V-a group element on the surface of said article. The method of this invention can be carried out without ageing the treating bath and can form a very smooth carbide layer on the surface of the article.

United States Patent [1 1 Komatsu et al.

[ Nov. 25, 1975 METHOD FOR FORMING OF A CARBIDE LAYER OF A V-A GROUP ELEMENT OF THE PERIODIC TABLE ON THE SURFACE OF AN IRON, FERROUS ALLOY OR CEMENTED CARBIDE ARTICLE Inventors: Noboru Komatsu, Toyoake; Tohru Arai; Yoshihiko Sugimoto, both of Nagoya; Masayoshi Mizutani, Nagoya, all of Japan Assignee: Kabushiki Kaisha Toyota Chuo Kenkyusho, Kagoya, Japan Filed: Apr. 27, 1973 Appl. No.: 355,230

Foreign Application Priority Data Apr. 12, 1973 Japan 48-40822 U.S. Cl. 427/399; 148/6.11; 148/155; 148/31.5; 427/431; 427/436; 428/457; 428/469 Int. Cl. C23C 9/10; C23F 7/00 Field of Search 117/114 R, 49, 51, 52, 117/113,130 R, 118, 160, 469 C, DIG. 11, 169 R; l48/6.11, 37,155, 30, 31.5; 123/439; 39/182.7

[56] References Cited UNITED STATES PATENTS 3,073,717 1/1963 Pyle et al. 117/DlG. 11 3,451,843 6/1969 Eckenrod... 117/130 R 3,600,284 8/1971 Martinez 204/39 3,719,518 3/1973 Komatsu et a1." 3,778,301

117/113 12/1973 Wohlberg 117/169 R Primary Examiner-Charles E. Van Horn Assistant Examiner-Michael W. Ball Attorney, Agent, or FirmWenderoth, Lind & Ponack [5 7 ABSTRACT A method for forming a carbide layer of a V-a group element V, Nb or TA of the periodic table on the surface of an iron, ferrous alloy or cemented carbide article in a treating molten bath, comprising heating a mixture of boric acid or a borate and a chloride of said V-a group element of the periodic table to its fusing state and immersing the article in the treating molten bath of said mixture, thereby forming a very hard carbide layer of said V-a group element on the surface of said article. The method of this invention can be carried out without ageing the treating bath and can form a very smooth carbide layer on the surface of the article.

9 Claims, 4 Drawing Figures US. Patent Nov. 25,

Sheet 1 of 2 :5: K mm; 5; an? E P 2 2i 9 E a: 8 a; m: E 2 m Z a 2 m nd M n. m a M m h 3 a W n3 IL IA 5 M 3 7 A 9 rd 1 5 S 2 0 N 2: m m w I METHOD FOR FORMING OF A CARBIDE LAYER OF A V-A GROUP ELEMENT OF THE PERIODIC TABLE ON THE SURFACE OF AN IRON, FERROUS ALLOY OR CEMENTED CARBIDE ARTICLE This invention relates to a method for forming a carbide layer of a V-a group element V,Nb or TA of the periodic table on the surface of an iron, ferrous alloy or cemented carbide article, and more particularly it relates to the formation of the carbide layer on the surface of the article immersed in a treating molten bath. The iron, ferrous alloy or cemented carbide article with the carbide layer formed thereon has a greatly improved hardness, wear resistance and machinability.

There have been reported several kinds of methods for coating or forming a metallic carbide layer on the surface of metallic articles. We have developed a method for forming a carbide layer of a V-a group element on the surface of a metallic article in a treating molten bath consisting of boric acid or a borate and a metal containing a V-a group element (Japanese Patent Application Ser. No. 44-87805). The method can form a uniform carbide layer and is highly productive and cheap. The carbide of a V-a group element, such as vanadium carbide (VC), niobium carbide (Nb) and tantalum carbide (TaC) has a very high hardness ranging from a Hv of 2000 to a Hv of 3000. Therefore, the carbide layer formed represents a high value of hardness and a superior resistance performance against wear and is thus highly suitable for the surface treatment of moulds such as dies and punches, tools such as pinchers and screwdrivers, parts for many kinds of tooling machines, and automobile parts to be subjected to wear.

Further, the carbide of a V-a group element is much harder and less reactive with iron or steel at a high temperature than the tungsten carbide forming cemented carbide is. Therefore, the formation of the carbide layer of a V-a group element on the surface of a cutting tool composed of a cemented carbide greatly increases the durability of the tool.

The method mentioned above requires a relatively long time for preparing the treating bath due to the slowness of dissolution of the treating metal particles. Sometimes the treating metal particles deposit into the carbide layer formed which makes the surface of the layer rough. I

Therefore, it is a principal object of this invention to provide an improved method for forming a carbide layer of a V-a group element on the surface of an iron, ferrous alloy or cemented carbide article in a treating molten bath.

It is another object of this invention to provide a method for forming a metallic carbide layer with denseness and uniformity and without any undissolved treating metal particles on the surface of the article.

It is still another object of this invention to provide a method for forming a carbide layer, which is safe and simple in practice and less expensive.

It is a still further object of this invention to provide a molten bath which does not necessitate ageing of the bath and is capable of forming a carbide layer having a smooth surface on the iron, ferrous alloy or cemented carbide article.

Other objects of this invention will appear hereinafter.

The novel features that are considered characteristic of the invention are set forth with particularity in the 2 appended claims. The invention, itself, as to its method of operation, together with additional objects and advantages therefore, will best be understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

FIG. 1 is a photomicrograph showing a vanadium carbide layer formed on the surface of a carbon tool steel according to Example 1;

FIG. 2 is a photomicrograph showing a vanadium carbide layer formed on the surface of a cemented carbide containing 9 percent by weight (hereinafter percent means percent by weight) of cobalt according to Example 3;

FIG. 3 is an X-ray diffraction chart of the layer shown in FIG. 2;

FIG. 4 is a photomicrograph showing a niobium carbide layer formed on a cemented carbide according to Example 4.

Broadly, the present invention is directed to an improvement of the method for forming a carbide layer of a V-a group element of the periodic table on the surface of an iron, ferrous alloy or cemented carbide article in a molten bath and is characterized in that the molten bath is composed of boric acid or a borate and a chloride of a V-a group element and in that the iron, ferrous alloy or cemented carbide article to be treated contains at least 0.05% of carbon. Namely, the method of the present invention comprises preparing a treating molten bath consisting of boric acid or a borate and a chloride of a V-a group element and immersing the iron, ferrous alloy or cemented carbide article into the treating molten bath so as to form the carbide layer on the surface of said article.

In the method of this invention, a chloride of a V-a group element is employed as a main ingredient of the treating molten bath instead of the powders of a V-a group element used in the previously developed method mentioned above. Said chloride is easily dissolved in a molten boric acid or borate and does not remain as solid particles. Therefore, the treating molten bath can be used as soon as the treating material is melted without the ageing of the bath and a very smooth carbide layer can be formed on the surface of the article.

In order to prepare the molten bath, a boric acid or a borate and a chloride of a V-a group element are mixed together and then the mixture is heated to its fusing state, or the boric acid or borate is heated to its fusing state and then the chloride is added into the molten boric acid or borate. As the chloride of a V-a group element, vanadium chloride (VCl VCl niobium chloride (NbCl and tantalum chloride (TaCl and the like can be used. As the borate, sodium borate (borax) (Na B O potassium borate (X 8 0 and the like can be used. In the treating molten bath, one or more than one kind of the chloride and borate, boric acid or a mixture of boric acid and a borate can be used. The boric acid and borate have the functions of dissolving metallic oxides and keeping the surface of the article to be treated clean. Also, the boric acid and borate are not poisonous and hardly vaporize. Therefore the method of the present invention can be carried out in the open air.

The chloride of a V-a group element may be included in the molten bath in a quantity between about 1 to 40%. With use of a less amount of the chloride than 1%, the formation of the carbide layer would not be uniform and would be formed too slowly for practical purposes. If the chloride is added in amounts more than 40%, the viscosity of the molten bath becomes too high to be normally operated and the corrosiveness of the molten bath becomes too strong.

The remainder of the treating molten bath is boric acid, a borate or mixtures thereof. Said boric acid or borate may be mixed in a quantity between 60 and 99%. In order to lower the viscosity of the molten bath, a salt such as a chloride or fluoride of an alkali metal can be added to the treating molten bath.

The iron, ferrous alloy or cemented carbide article to be treated must contain at least 0.05% of carbon, and should preferably contain 0.1% of carbon or higher. The carbon in the article forms a carbide during the treatment. Namely it is supposed that the carbon in the article diffuses to the surface thereof and reacts with the V-a group element from the treating molten bath to form the carbide on the surface of the article. A higher content of the carbon in the article is more preferable for forming the carbide layer. The iron, ferrous alloy or cemented carbide article containing less than 0.05% of carbon may not be formed with a uniform and thick carbide layer by treatment. Also, the article containing at least 0.05% of carbon only in the surface portion thereof can be treated to form a carbide layer on the surface of the article. For example, a pure iron article, which is casehardened to increase the carbon content in the surface portion thereof, can be used as the article of the present invention.

l-lere, iron means iron containing carbon and casehardened iron; ferrous alloy means carbon steel and alloy steel, and cemented carbide means a sintered tungsten carbide containing cobalt. Said cemented carbide may include a small amount of titanium carbide, niobium carbide, tantalum carbide and the like.

In some cases, the carbon contained in the treating molten bath can be used as the source of the carbon for forming the carbide layer on the surface of the article. However, the formation of the carbide layer is not stable and the use of the carbon in the treating molten bath is not practical.

Before the treatment, it is important to purify the surface of the article for forming a good carbide layer by washing out the rust and oil from the surface of the article with an acidic aquious solution or another liquid.

The treating temperature may be selected within a wide range from the melting point of boric acid or borate to the melting point of the article to be treated. Preferably, the treating temperature may be selected within the range from 800 to 1,lC. With lowering of the treating temperature, the viscosity of the treating molten bath increases gradually and the thickness of the carbide layer formed decreases. However, at a relatively high treating temperature, the quality of the molten bath is deteriorated. Also the quality of the material forming the article is deteriorated by increasing the crystal grain sizes of said material.

The treating time depends upon the thickness of the carbide layer to be formed. Heating shorter than minutes will, however, provide no practically accepted formation of said layer, although the final determination of the treating time depends on the treating temperature. With the increase of the treating time, the thickness of the carbide layer will be increased accordingly. In practice, an acceptable thickness of the layer can be realized within 30 hours or shorter. The prefera- 4 ble range of the treating time is from 10 minutes to 30 hours.

The vessel for maintaining the treating molten bath of the present invention can be made of graphite or heat resistant steel.

It is not necessary to carry out the method of the present invention in a of non-oxidation gas atmosphere, since the method can be carried out either under air or in an inert gas atmosphere.

EXAMPLE 1 100 grams of borax was introduced into a graphite crucible having an inner diameter of 35mm innerdiameter and heated up to 900C for melting the borax in an electric furnace in the air, and then 16 grams of vanadium chloride (VCl powder was poured into the molten borax and mixed together for preparing a treating molten bath. Next, a specimen, 5mm diameter and 40mm long, made of carbon tool steel (JlS 8K4, containing 1.0% of carbon) was immersed into the treating molten bath and kept therein for 2 hours, taken out therefrom and air-cooled. Treating material which has adhered to the surface of the specimen was removed by washing with hot water and then the specimen treated was investigated. The surface of the specimen was very smooth. After cutting and polishing the specimen, the specimen was micrographically observed, and it was found that a layer as shown in FIG. 1 was formed. The thickness of the layer was about 7 microns. The layer was identified to be vanadium carbide (VC) by an X-ray diffraction method and by an X-ray microanalyzer. Boron was not detected from the specimen treated.

EXAMPLE 2 700 grams of borax was introduced into a graphite crucible having an inner diameter of mm and heated up to 950C for melting the borax in an electric furnace under air, and then 120 grams of niobium chloride powder was poured into the molten borax and mixed together for preparing a treating molten bath. Next, a specimen, 8mm in diameter and 40mm long, made of tool alloy steel (JIS SKD61, containing 0.45% of carbon) was immersed into the treating molten bath and kept therein for 2 hours, taken out therefrom and aircooled. Treating material adhered to the surface of the specimen was removed by washing with hot water. The

surface of the specimen treated was very smooth. After cutting and polishing the specimen, the cross section of the specimen was micrographically observed and tested by an X-ray diffraction method and by an X-ray microanalyzer. The layer formed was identified to be niobium carbide and the thickness of the layer was about 4 microns.

EXAMPLE 3 grams of borax powder was introduced into a graphite crucible and heated up to l,00OC for melting the borax in an electric furnace under air, and then 38 grams of vanadium chloride (VCl powder of less than 100 mesh was poured into the molten borax together with mixing. Next, a specimen, lmm thick, 5.5mm wide and 30mm long made of cemented carbide composed of 91% of tungsten carbide and 9% of cobalt was immersed into the treating molten bath and kept therein for 15 hours, taken out therefrom and air-cooled. Treating material adhered to the surface of the specimen was removed by dipping the specimen into hot water. The surface of the specimen treated was smooth.

After cutting and polishing the specimen, the cross sectional area of the specimen was micrographically observed and tested by an X-ray diffraction method and by an X-ray microanalyzer. A layer as shown in FIG. 2 was found. By an X-ray diffraction method, strong vanadium carbide (VC) diffraction lines were detected from the layer. In FIG. 3, the chart of the X-ray diffraction is shown. By use of an X-ray microanalyzer, the layer was found to contain a large amount of vanadium. The Hv hardness (Micro-Vickers Hardness) of the layer measured from the surface of the specimen was found to be about 2983. Also the Hv hardness (Micro- Vickers Hardness) of the mother material of the specimen was measured to be about 1525.

EXAMPLE 4 In the same manner as described in Example 3, a treating molten bath composed of 100 grams of borax and 25 grams of niobium chloride was prepared. Then a specimen having the same sizes and made of the same material as the specimen in Example 3 was treated for 4 hours at l,000C. By the treatment, a layer shown in FIG. 4 was formed on the surface of the specimen. Also the layer was tested by an X-ray diffraction method, an X-ray microanalyzer and Vickers Hardness Tester. Strong niobium carbide (NbC) diffraction lines were detected. The layer was found to contain a large amount of niobium, and the Hv hardness of the layer was about 2750.

What is claimed is:

1. A method for forming a carbide layer on the surface of an iron, ferrous alloy or cemented carbide article in a molten bath, comprising the steps of preparing the molten bath consisting essentially of a chloride of an element selected from the group consisting of vanadium, niobium, tantalum and mixtures thereof and one member selected from the group consisting of boric 6 acid, a borate and mixtures thereof, immersing the article containing at least 0.05 percent of carbon into said molten bath, maintaining said article in said molten bath for forming the carbide layer of said element on the surface of said article, and removing the article out of the molten bath.

2. A method according to claim 1, wherein said borate is sodium borate or potassium borate.

3. A method according to claim 1, wherein said article is immersed into the molten bath composed of l to 40 percent of said chloride and the balance of being said boric acid or borate and maintaining the article in the molten bath for 10 minutes to 30 hours at a temperature ranging from 800 to l,l0OC.

4. A method according to claim 1, wherein said article is a ferrous alloy selected from the group consisting of carbon steel and alloy steel containing at least 0.05 percent of carbon.

5. A method according to claim 1, wherein said article is a cemented carbide article.

6. A method according to claim 1, wherein the step of preparing the molten bath comprises heating boric acid or the borate up to its fusing state, adding the chloride into said molten boric acid or borate and mixing said chloride and said molten boric acid or borate.

7. A method according to claim 1, wherein the step of preparing the molten bath comprises preparing the mixture of the chloride and boric acid or borate and heating said mixture up to its fusing state.

8. A method according to claim 1, wherein said molten bath contains chloride or fluoride of an alkali metal for lowering the viscosity of the molten bath.

9. A method according to claim 6, wherein said cemented carbide article is made of a sintered tungsten carbide containing cobalt. 

1. A METHOD FOR FORMING A CARBIDE LAYER ON THE SURFACE OF AN IRON, FERROUS ALLOY OR CEMENTED CARBIDE ARTICLE IN A MOLTEN BATH, COMPRISING THE STEPS OF PREPARING THE MOLTEN BATH CONSISTING ESSENTIALLY OF A CHLORIDE OF AN ELEMENT SELECTED FROM THE GROUP CONSISTING OF VANADIUM,NIOBIUM, TANTALUM AND MIXTURES THEREOF AND ONE OEMBER SELECTED FROM THE GROUP CONSISTING OF BORIC ACID, A BORATE AND MIXTURES THEREOF, IMMERSING THE ARTICLE CONTAINING AT LEAST 0.05 PERCENT OF CARBON INTO SAID MOLTEN BATH, MAINTAINING SAID ARTICLE IN SAID MOLTEN BATH FOR
 2. A method according to claim 1, wherein said borate is sodium borate or potassium borate.
 3. A method according to claim 1, wherein said article is immersed into the molten bath composed of 1 to 40 percent of said chloride and the balance of being said boric acid or borate and maintaining the article in the molten bath for 10 minutes to 30 hours at a temperature ranging from 800 to 1,100*C.
 4. A method according to claim 1, wherein said article is a ferrous alloy selected from the group consisting of carbon steel and alloy steel containing at least 0.05 percent of carbon.
 5. A method according to claim 1, wherein said article is a cemented carbide article.
 6. A method according to claim 1, wherein the step of preparing the molten bath comprises heating boric acid or the borate up to its fusing state, adding the chloride into said molten boric acid or borate and mixing said chloride and said molten boric acid or borate.
 7. A method according to claim 1, wherein the step of preparing the molten bath comprises preparing the mixture of the chloride and boric acid or borate and heating said mixture up to its fusing state.
 8. A method according to claim 1, wherein said molten bath contains chloride or fluoride of an alkali metal for lowering the viscosity of the molten bath.
 9. A method according to claim 6, wherein said cemented carbide article is made of a sintered tungsten carbide containing cobalt. 